Various methods for reducing the amount of carbon monoxide in the mainstream smoke of a cigarette during smoking have been proposed. For example, British Patent No. 863,287 describes methods for treating tobacco prior to the manufacture of tobacco articles, such that incomplete combustion products are removed or modified during smoking of the tobacco article. This is said to be accomplished by adding a calcium oxide or a calcium oxide precursor to the tobacco. Iron oxide is also mentioned as an additive to the tobacco.
Cigarettes comprising absorbents, generally in a filter tip, have been suggested for physically absorbing some of the carbon monoxide, but such methods are usually not completely efficient. A cigarette filter for removing unwanted byproducts formed during smoking is described in U.S. Re. Pat. No. RE 31,700, where the cigarette filter comprises dry and active green algae, optionally with an inorganic porous adsorbent such as iron oxide. Other filtering materials and filters for removing unwanted gaseous byproducts, such as hydrogen cyanide and hydrogen sulfide, are described in British Patent No. 973,854. These filtering materials and filters contain absorbent granules of a gas-adsorbent material, impregnated with finely divided oxides of both iron and zinc. In another example, an additive for smoking tobacco products and their filter elements, which comprises an intimate mixture of at least two highly dispersed metal oxides or metal oxyhydrates, is described in U.S. Pat. No. 4,193,412. Such an additive is said to have a synergistically increased absorption capacity for toxic substances in the tobacco smoke. British Patent No. 685,822 describes a filtering agent that is said to oxidize carbon monoxide in tobacco smoke to carbonic acid gas. This filtering agent contains, for example, manganese dioxide and cupric oxide, and slaked lime. The addition of ferric oxide in small amounts is said to improve the efficiency of the product.
The addition of an oxidizing reagent or catalyst to the filter has been described as a strategy for reducing the concentration of carbon monoxide reaching the smoker. The disadvantages of such an approach, using a conventional catalyst, include the large quantities of oxidant that often need to be incorporated into the filter to achieve considerable reduction of carbon monoxide. Moreover, if the ineffectiveness of the heterogeneous reaction is taken into account, the amount of the oxidant required would be even larger. For example, U.S. Pat. No. 4,317,460 describes supported catalysts for use in smoking product filters for the low temperature oxidation of carbon monoxide to carbon dioxide. Such catalysts include mixtures of tin or tin compounds, for example, with other catalytic materials, on a microporous support. Another filter for smoking articles is described in Swiss patent 609,217, where the filter contains tetrapyrrole pigment containing a complexed iron (e.g. haemoglobin or chlorocruorin), and optionally a metal or a metal salt or oxide capable of fixing carbon monoxide or converting it to carbon dioxide. In another example, British Patent No. 1,104,993 relates to a tobacco smoke filter made from sorbent granules and thermoplastic resin. While activated carbon is the preferred material for the sorbent granules, it is said that metal oxides, such as iron oxide, may be used instead of, or in addition to the activated carbon. However, such catalysts suffer drawbacks because under normal conditions for smoking, catalysts are rapidly deactivated, for example, by various byproducts formed during smoking and/or by the heat. In addition, as a result of such localized catalytic activity, such filters often heat up during smoking to unacceptable temperatures.
Catalysts for the conversion of carbon monoxide to carbon dioxide are described, for example, in U.S. Pat. Nos. 4,956,330 and 5,258,330. A catalyst composition for the oxidation reaction of carbon monoxide and oxygen to carbon dioxide is described, for example, in U.S. Pat. No. 4,956,330. In addition, U.S. Pat. No. 5,050,621 describes a smoking article having a catalytic unit containing material for the oxidation of carbon monoxide to carbon dioxide. The catalyst material may be copper oxide and/or manganese dioxide. The method of making the catalyst is described in British Patent No. 1,315,374. Finally, U.S. Pat. No. 5,258,340 describes a mixed transition metal oxide catalyst for the oxidation of carbon monoxide to carbon dioxide. This catalyst is said to be useful for incorporation into smoking articles.
Metal oxides, such as iron oxide have also been incorporated into cigarettes for various purposes. For example, in WO 87/06104, the addition of small quantities of zinc oxide or ferric oxide to tobacco is described, for the purposes of reducing or eliminating the production of certain unwanted byproducts, such as nitrogen-carbon compounds, as well as removing the stale “after taste” associated with cigarettes. The iron oxide is provided in particulate form, such that under combustion conditions, the ferric oxide or zinc oxide present in minute quantities in particulate form is reduced to iron. The iron is claimed to dissociate water vapor into hydrogen and oxygen, and cause the preferential combustion of nitrogen with hydrogen, rather than with oxygen and carbon, thereby preferentially forming ammonia rather than the unwanted nitrogen-carbon compounds.
In another example, U.S. Pat. No. 3,807,416 describes a smoking material comprising reconstituted tobacco and zinc oxide powder. Further, U.S. Pat. No. 3,720,214 relates to a smoking article composition comprising tobacco and a catalytic agent consisting essentially of finely divided zinc oxide. This composition is described as causing a decrease in the amount of polycyclic aromatic compounds during smoking. Another approach to reducing the concentration of carbon monoxide is described in WO 00/40104, which describes combining tobacco with loess and optionally iron oxide compounds as additives. The oxide compounds of the constituents in loess, as well as the iron oxide additives are said to reduce the concentration of carbon monoxide.
Moreover, iron oxide has also been proposed for incorporation into tobacco articles, for a variety of other purposes. For example, iron oxide has been described as particulate inorganic filler (e.g. U.S. Pat. Nos. 4,197,861; 4,195,645; and 3,931,824), as a coloring agent (e.g. U.S. Pat. No. 4,119,104) and in powder form as a burn regulator (e.g. U.S. Pat. No. 4,109,663). In addition, several patents describe treating filler materials with powdered iron oxide to improve taste, color and/or appearance (e.g. U.S. Pat. Nos. 6,095,152; 5,598,868; 5,129,408; 5,105,836 and 5,101,839). However, the prior attempts to make cigarettes incorporating metal oxides, such as FeO or Fe2O3 have not led to the effective reduction of carbon monoxide in mainstream smoke.
Despite the developments to date, there remains a need for improved and more efficient methods and compositions for reducing the amount of carbon monoxide in the mainstream smoke of a cigarette during smoking. Preferably, such methods and compositions should not involve expensive or time consuming manufacturing and/or processing steps. More preferably, it should be possible to catalyze or oxidize carbon monoxide not only in the filter region of the cigarette, but also along the entire length of the cigarette during smoking.